What is PostGIS?

PostGIS is an extension to the ​PostgreSQL object-relational database system that allows ​GIS (Geographic Information System) objects to be stored in the database.
PostGIS does for PostgreSQL what Oracle Spatial does for Oracle, ArcSDE does for Microsoft SQL Server/Oracle.

PostGIS Books

​PostGIS in Action (2nd Edition) by Regina Obe and Leo Hsu - Covers PostGIS 2.1. Feature complete and available from Manning in draft form. With early purchase you get draft updates and final (e-Book/Hardcover?) when complete. E-book is generally 1/2 price of full. All code and data downloads available for free on the book site even if you don't buy the book. ​Chapter 1 (a primer on PostGIS ) are free downloads.

​PostGIS in Action (1st Edition) by Regina Obe and Leo Hsu - It's out and available via various distributors including Manning Publications and Amazon. If you want just the E-Book, purchase directly from Manning. The E-book is about 50% off the hard-copy price. With the purchase of Hard-copy, there is a free-coupon to get the E-Book version. Book includes PostGIS 1.3-1.5, PostgreSQL 8.3-9.0 (with upcoming highlights of PostgreSQL 9.1 and PostGIS 2.0). All code and data downloads available for free on the book site even if you don't buy the book. ​Chapter 1 (a primer on PostGIS ) and ​Appendix C (a primer on SQL) are free downloads.